At the library of congress, the largest library in the world I discovered a interesting feature (which a lot may already know): If you search for a book using the "basic search feature, you can click on the "call number" of the book ("Signatur" in german). Now you can browse up and down through the books with a call number near the searched book and you will find mostly books with a very similar topic (but not always). A great help, even if you have to order the books at your local library afterwards.

The call numbers are sorted by an hierachical classifacation system, which is best explained at this external site. Also you may want to read more about the composition of a call number.

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